What is required for a water pump for a vehicle for circulating coolant to each component of a vehicle, for example to an internal-combustion engine, is to vary a supplying amount of the coolant as needed by applying a clutch mechanism to the water pump.
For example, a known water pump disclosed in JP11-201191A includes a clutch mechanism configured by a wrap spring arranged about an input hub and an output shaft, a friction member connected to one of the input hub and the output shaft for rotating therewith, an armature connected to the wrap spring and an actuator for selectively causing the friction member and the armature to frictionally engage each other, where the clutch mechanism selectively causes the wrap spring to frictionally engage the input hub or the output shaft in accordance with the frictional engagement between the friction member and the armature.
Further, another known water pump disclosed in JPH8-002423Y includes a clutch mechanism configured by a solenoid, a pulley, an inner coupling member, an outer coupling member, an armature plate and a friction plate. The solenoid is fixed on an outer circumferential surface of an end portion of a bearing housing by a bracket. The pulley is freely rotatably supported via a bearing at an outer circumferential surface of an end portion of a driving shaft of the water pump, the driving shaft being freely rotatably inserted into the bearing housing. The inner coupling member is freely rotatably supported via the bearing at the outer circumferential surface of the end portion of the driving shaft. The outer coupling member, arranged so as to face the inner coupling member via viscosity fluid, is fixed at the outer circumferential surface of the end portion of the driving shaft. The armature plate, made of a magnetic material and facing the solenoid via the pulley, is fixed at the outer coupling member. The friction plate is attached to a side surface of the armature plate at the side of the pulley.
Because each of the abovementioned clutch mechanisms has a complicated structure and is large in size, in a case where such clutch mechanism is applied to the water pump, the size of the pump itself may be increased.
According to the clutch mechanism disclosed in JP11-201191A, because some components are provided between the actuator and the armature, a structure of the clutch mechanism becomes complicated, accordingly a number of air gaps need to be provided. However, because of the air gaps, electromagnetic force generated by the actuator partially acts so as to attract the armature, accordingly the level of affectivity of the electromagnetic force may be decreased.
Furthermore, according to the electromagnetic clutch disclosed in JPH8-002423Y, a clutch force used for a torque transmission is set depending on the attracting force of the solenoid. In a case where such electromagnetic clutch is applied to the water pump, because the level of the clutch force needs to be increased when the engine is rotated in a high speed, power of the solenoid needs to be increased. In a case where a large electromagnetic clutch is actuated, high electric current is applied to the electromagnetic clutch, and such water pump is increased in size, thereby deteriorating mountability to the vehicle and increasing costs thereof.
A need thus exists to provide a water pump, which is not susceptible to the drawback mentioned above.